This application claims the priority of German Application No. 100 12 988.8, filed Mar. 16, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for operating an electromagnetic actuator and, more particularly, to a process for actuating a gas exchange lift valve of an internal combustion engine, with an armature, which is moved oscillatingly between two electromagnetic coils against the force of at least one return spring via an alternating supply of current to the electromagnetic coils.
A preferred application for an electromagnetic actuator of the type described above is for electromagnetically actuating a valve drive mechanism of an internal combustion engine. That is, the gas exchange lift valves of an internal combustion piston engine are actuated by such actuators in the desired manner, so as to be opened and closed in an oscillating manner. In such an electromechanical valve drive mechanism, the lift valves are moved individually (or also in groups) by means of electromechanical actuating elements - the so-called actuators - whereby the time for opening and closing each lift valve can be selected in essence arbitrarily. Thus, the valve timing of the internal combustion engine can be adjusted optimally to the current operating state (which is defined by the speed and the load) and to the respective requirements with respect to consumption, torque, emission, comfort and response characteristics of a motor vehicle, driven by the internal combustion engine.
The essential components of a known actuator for actuating the lift valves of an internal combustion engine are an armature and two electromagnets for holding the armature in the position xe2x80x9clift valve openxe2x80x9d or xe2x80x9clift valve closedxe2x80x9d with the related electromagnetic coils. Furthermore, return springs are provided for the movement of the armature between the position xe2x80x9clift valve openxe2x80x9d and xe2x80x9clift valve closedxe2x80x9d. In this respect reference is also made to the attached FIG. 1, which depicts such an actuator with a related lift valve in the two possible end positions of the lift valve and the actuator-armature. Between the two illustrated states or positions of the actuator lift valve unit, diagrams illustrate the curve of the armature lift z or the armature path between the two electromagnetic coils and, furthermore, the current flow I in the two electromagnetic coils over time t in accordance with the known state of the art (which is simpler than the mechanism described in German Patent document DE 195 30 121 Al, discussed in the introductory part of the specification).
FIG. 1 depicts the closing operation of an internal combustion engine lift valve, which is marked with the reference numeral 1. As usual, a valve closing spring 2a acts on the lift valve 1. Furthermore, the actuator, which is generally designated by reference numeral 4 in its entirety, acts on the shaft of the lift valve 1 - here with intercalation of a hydraulic valve play compensating element 3 (which is not absolutely necessary). The actuator 4 comprises not only two electromagnetic coils 4a, 4b, but also a push rod 4c, which acts on the shaft of the lift valve 1 and which bears an armature 4d. The armature 4d can be slid longitudinally and oscillatingly between the electromagnetic coils 4a, 4b. Furthermore, a valve opening spring 2b acts on the end of the push rod 4c, facing away from the shaft of the lift valve 1.
Thus, FIG. 1 depicts an oscillatory system, for which the valve closing spring 2a and the valve opening spring 2b form a first and a second return spring, for which consequently the reference numerals 2a, 2b are also used.
The first end position of this oscillatory system is shown on the left hand side of FIG. 1, where the lift valve 1 is completely open and the armature 4d rests against the bottom electromagnetic coil 4b. This coil 4b is also called hereinafter the opener coil 4b, since it holds the lift valve 1 in its opened position.
The second end position of the oscillatory system is shown on the right hand side of FIG. 1, where the lift valve 1 is completely closed and the armature 4d rests against the upper electromagnetic coil 4a. This coil 4a is also called hereinafter the closer coil 4a, since it holds the lift valve 1 in its closed position.
At this point, the closing operation of the lift valve 1 will now be described, that is, in FIG. 1 the transition from the open state, illustrated on the left hand side, into the closed state, illustrated on the right hand side. Between the two sides the corresponding curves of the electrical currents I, flowing into the coils 4a, 4b, and the lift curve or the path coordinate z of the armature 4d are plotted, respectively, over time t.
Starting from the left-hand side position xe2x80x9clift valve openxe2x80x9d, the supply of current is guided first to the opener coil 4b so that the armature 4d pushes in this position against the stressed valve closing spring 2a (=bottom first return spring 2a), whereby the current I in this coil 4b is shown with a dashed line in the I-t diagram. If at this stage the current I of the opener coil 4b is turned off for a desired transition to xe2x80x9clift valve closedxe2x80x9d, the armature 4d detaches from this coil 4b and the lift valve 1 is accelerated by means of the stressed valve closing spring 2a into approximately its central position (in the direction toward the top of the page), but then continues to move owing to its mass inertia so as to thereby stress the valve opening spring 2b, so that the lift valve 1 (and the armature 4d) are decelerated. Then, at an appropriate time, the supply of current is guided to the closer coil 4a (the current I for the coil 4a is shown with a solid line in the I-t diagram). Thus, this coil 4a xe2x80x9ccatchesxe2x80x9d the armature 4d (this operation is the so-called xe2x80x9ccatchxe2x80x9d process), and holds it finally in the position xe2x80x9clift valve closedxe2x80x9d, illustrated on the right hand side of FIG. 1. After the armature 4d has been securely caught by the coil 4a, the current in this coil is switched over, moreover, to a lower holding current level (see I-t diagram).
Starting from the position, illustrated on the right hand side in FIG. 1, the reverse transition from xe2x80x9clift valve closedxe2x80x9d to xe2x80x9clift valve openxe2x80x9d takes place analogously. The current I in the closer coil 4a is turned off and the current for the opener coil 4b is turned on with a time delay. Generally, for the supply of current to be guided to the coils 4a, 4b, sufficient electric voltage is applied to said coils, whereas the turning off of the electric current I is triggered by lowering the electric voltage to the value xe2x80x9czeroxe2x80x9d. The necessary electric energy for operating each actuator 4 is taken either from the electrical system of the vehicle, driven by the related internal combustion engine, or provided by means of a separate energy supply, adjusted to the valve drive mechanism of the internal combustion engine. In this respect the electric voltage is held constant by the energy supply; and the coil current I of the actuators 4, assigned to the internal combustion engine lift valves 1, is controlled in such a manner by a controller that the necessary forces for the opening, closing and holding of the lift valve(s) 1 in the desired position are generated.
In the state of the art, described above, the aforementioned controller or a control unit adjusts through timing the coil current I during the so-called catch process (wherein one of the two coils 4a, 4b endeavors to catch the armature 4d) to a value that is large enough to catch reliably the armature 4d under all conditions. Now the force of the catching electromagnetic coil 4a or 4b on the armature 4d is approximately proportional to the current I and inversely proportional to the distance between the coil and the armature. If at this stagexe2x80x94as in the known state of the art xe2x80x94a constant current I is set, the magnetic force, acting on the armature 4d, increases, as it approaches the respective coil 4a or 4b, catching it, inversely proportional to the remaining gap, thus increasing the acceleration and speed of the armature. The result is a high landing speed of the armature 4b on the respective electromagnetic coil 4a or 4b, the consequence of which is, first of all, high wear in the actuator 4 and, secondly, also high significant noise development. Another drawback lies in the change over losses of the transistors, which losses are generated during the briefly described timed current regulation and which result in increased power absorption and temperature load on the controller that is used and increased electromagnetic radiation into the leads of the actuators.
The state of the art, disclosed in German Patent document DE 195 30 121 A1 discussed above, does offer an improvement especially with respect to the noise development and the actuator wear. It proposes a process for reducing the landing speed of the armature on an electromagnetic actuator. As the armature approaches the pole surface of the coil catching the armature, the voltage, applied to this coil, is limited (that is, essentially reduced) to a specified maximum value so that the current, flowing through the coil, drops during a part of the time that the voltage is limited. Furthermore, it is also stated that the degree to which the voltage is limited or reduced can be specified in a family of characteristics. The corresponding values and, in particular, also the respective time at which this voltage reduction is supposed to start, can be determined experimentally.
The German Patent document DE 198 32 198 A1 describes a process for reducing the landing speed of an armature on the electromagnetic actuator, where in the braking phase, which follows a catch phase, a timed electric voltage is applied. In so doing, the respective switching times and the voltage-to-timing ratio of a regulator are determined with the aid of a desired trajectory, describing the desired movement of the armature.
The actual voltage curves for the implementation of this process are usually found with so-called controller based design methods. They involve empirical or numerical, thus arithmetic methods, with which a voltage curve is determined when specific boundary conditions are set and with which the desired result can be shown.
However, these past methods did not take into consideration that from time to time there are high current eddies in the armature. These eddies result in energy losses and inaccuracies in the individual valve timing. In the past these drawbacks were remedied in that the current eddies were largely avoided through the use of a laminated armature or an armature with low electric conductivity. However, such solutions usually lead to higher costs.
The object of the present invention is therefore to provide a process in which the high current eddies in the armature can be largely avoided during the operation of an electromagnetic actuator.
This problem is solved according to the invention by a process for operating an electromagnetic actuator, in particular to actuate a gas exchange lift valve of an internal combustion engine, with an armature, which is moved oscillatingly between two electromagnetic coils against the force of at least one return spring via an alternating supply of current to the electromagnetic coils and whereby, as the armature approaches the coil subjected first to a current flow, during the so-called catch process the voltage, applied to the coil catching the armature, is reduced, characterized in that a voltage control method is chosen with a voltage curve, with which an eddy current, calculated with a mathematical model, is minimized in the armature.
An important idea of the present invention is the consideration of the eddy currents, induced in the armature, during preparation of the voltage curve to regulate/control the electromagnetic actuator. From a number of possible voltage curves, which allow the operation of an electric actuator in the required manner, a voltage curve can be selected in the sense of avoiding eddy currents.
To understand the invention it is helpful to design an equivalent circuit diagram for the magnetic fluxes that shows the magnetic sources and the magnetic resistances. In this equivalent circuit diagram the eddy current, induced in the armature, represents a magnetic flux source that generates a magnetic flux, which flows in the opposite direction to the other magnetic flux. In considering this magnetic flux source in a mathematical model, with which the current eddies in the armature can be calculated, a voltage control method can be chosen that results in a minimum eddy current.
Preferably, a mathematical or numerical model is used to find the voltage curve. One possibility for determining the voltage curve lies in the use of a so-called controller design method with variable structure, as is well known.
The voltage curve can then be realized by different ways and means, for example through pulse width modulation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.